1. Field of the Invention
The present invention relates to a MOS type solid-state image pickup apparatus and a method of manufacturing the same and, more particularly, to a MOS type solid-state image pickup apparatus capable of picking up an image of high quality and a method of manufacturing the same.
2. Description of the Related Art
FIG. 6A is a schematic view of a surface of a CMOS image sensor 1 constituted by a plurality of photodiodes (photoelectric conversion elements) arranged in the form of a square grid on a light-receiving surface (image area) of a semiconductor substrate. FIG. 6B is a circuit diagram of the same. In the illustrated image sensor 1, a multiplicity of unit pixels 3 are arranged on a light-receiving surface 2; a control pulse generating circuit 4 and a vertical scanning circuit 5 are formed on a side of the light-receiving surface 2; and a noise suppression circuit 6 and a horizontal scanning circuit 7 are formed along a lower edge of the light-receiving surface 2.
Characters R, G, and B shown on the unit pixels 3 in FIG. 6A indicate red filters (R), green filters (G), and blue filters (B) formed on the photodiodes constituting the unit pixels.
A unit pixel 3 is constituted by a photodiode 3a (see FIG. 6B) and a signal reading circuit for reading a signal detected by the photodiode 3a (although the circuit is shown in FIG. 6B as a known signal reading circuit constituted by four transistors, a configuration including three transistors may alternatively be used).
Wirings 10 extending in an X-direction (horizontal direction) and wirings 11 extending in a Y-direction (vertical direction) are laid on the light-receiving surface 2 of the CMOS image sensor 1. The wirings 10 are connected to the control pulse generating circuit 4 and the vertical scanning circuit 5, and the wirings 11 are connected to the noise suppression circuit 6, the horizontal scanning circuit 7, and a power supply which is not shown.
The wirings 10 and 11 laid on the light-receiving surface 2 in the X- and Y-direction will be hereinafter referred to as “global wirings” to distinguish them from, for example, internal wirings of a signal reading circuit and internal wirings of the control pulse generating circuit 4, the vertical scanning circuit 5, the noise suppression circuit 6, and the horizontal scanning circuit 7. The global wirings include a row select line, a row reset line, a power supply line, and output signal line, and they are formed from a metal such as aluminum or copper, in general.
It is said that the manufacturing cost of such a CMOS image sensor 1 according to the related is low because it can be manufactured using general CMOS processes (DRAM processes) unlike a CCD image sensor for which dedicated manufacturing processes are used.
The reason is that a part (p-n junction) of a MOS transistor manufactured in the same way as other CMOS-LSIs is used as the photodiode 3a of the CMOS image sensor 1 and that a signal reading circuit for reading a signal from the photodiode 3a is also constructed as a combination of a plurality of MOS transistors.
A photodiode must be selected from among the photodiodes 3a to read a signal from the same, and such selection can be made through the global wirings 10 connected to the signal reading circuit of each photodiode just like the selection of a memory element such as a DRAM.
FIG. 7A is a schematic perspective view of one unit pixel of the CMOS image sensor, and FIG. 7B is a schematic view of a section of the same. Visible light rays 15 enter each unit pixel from the outside through a micro-lens (top lens) 16 and a color filter layer 17, and the light reaches the photodiode 3a. 
At this time, when the global wiring 10 extending in the X-direction and the global wiring 11 extending in the Y-direction block a part of the incident light, multiple reflection of the part of the incident light occurs between the global wirings 10 and 11 or between the wirings and a metal film (shield film which is normally an aluminum thin film) for shielding the signal reading circuit 18 (see FIG. 7A) except the photodiode 3a from light. When resultant multiple reflection light 20 leaks into an adjacent photodiode 3a, a problem arises in that an image thus picked up has low quality.
The photodiode 3a formed on the semiconductor substrate is separated from a MOS transistor constituting a signal reading circuit for selecting and amplifying a signal by a device separating region 21. At a CMOS process, a gate electrode 22 forming a part of the MOS transistor is formed in the device separating region 21; a leveling protective film 23 is formed on the electrode; and a first wiring layer which is a metal film such as aluminum is formed thereafter.
Let us assume that the first wiring layer is the global wiring 10 extending in the X-direction. Then, it is required to form a leveled insulation film further on the global wiring 10 in the X-direction such that the global wiring 11 in the Y-direction and the global wiring 10 will not cross and electrically short with each other and to form the global wiring 11 in the Y-direction on the film.
Normally, another leveling film is formed on the wiring and a shielding layer 19 is formed thereon. Further, another leveling film is formed on the layer and a color filter layer 17 is formed thereon. As thus described, global wirings are generally formed in a multi-layer structure at a CMOS process.
Referring to the material of the global wirings, a low-resistance metal material such as Al (aluminum) is normally used to ensure that a resultant integrated circuit (IC) operates at a high speed. However, the use of aluminum makes it difficult to solve the above-mentioned problem of multiple reflections because it has a high surface reflectivity.
For example, techniques associated with CMOS image sensors in the related art include that disclosed in JP-A-2001-298176.
A MOS image sensor (MOS type solid-state image pickup apparatus) is constructed by forming wiring layers in a multi-layer structure on a semiconductor substrate having photodiodes and signal reading circuits formed thereon and forming optical layers such as color filters and micro-lenses on the wiring layers. The recent progress in micro-processing techniques has resulted in a trend toward image sensors having a greater number of pixels (a higher pixel density). Thus, the dimensions of the aperture of one pixel are becoming smaller, and the distance between adjoining pixels is also becoming smaller. On the contrary, efforts toward the reduction of the height of pixels have been unsuccessful, and an optical path traveled by light incident on a micro-lens of each pixel to reach a photodiode is becoming longer and narrower. The effect of multiple reflections is thus becoming no longer ignorable as the trend toward image sensors with a greater number of pixels proceeds.